1. Field of the Invention
The present invention relates to a magnetic recording medium, particularly to a magnetic recording medium each having a magnetic layer with a smooth surface, being capable of carrying out electromagnetic conversion further satisfactorily and exhibiting a reduced error rate.
2. Description of the Related Art
Magnetic recording medium typically for use in audio-visual apparatuses and computers have become more and more denser in their recording density. Such magnetic recording medium must have a reduced minimal recording unit. For this reason, a recording wavelength is shortened, and the recording track width and the thickness of recording medium are reduced. For adapting these requirements, fine powders of ferromagnetic metals each having large magnetic energy have been used in the magnetic recording medium.
With an increasing recording density of such a magnetic recording medium, the magnetic layer of the magnetic recording medium must have a further reduced thickness and a further smoothened surface. Most of magnetic recording medium exhibiting a high recording density each have a multilayer structure and comprise a non-magnetic substrate, a non-magnetic layer and a magnetic layer. The non-magnetic layer is arranged on or above one side of the non-magnetic substrate and contains a non-magnetic powder, a carbon black and a binder resin. The magnetic layer is arranged on or above the non-magnetic layer and contains a ferromagnetic metal powder and a binder resin. The upper magnetic layer of the magnetic recording medium becomes thinner and thinner, and the surface smoothness of the magnetic layer is thus largely affected by the surface smoothness of the lower non-magnetic layer.
Attempts have been made to smoothen a surface of the non-magnetic layer by highly dispersing a non-magnetic powder in the non-magnetic layer to improve the surface smoothness of the magnetic layer. Japanese Unexamined Patent Application Publication No. 2002-25036, for example, discloses a magnetic recording medium prepared by applying coating compositions comprising a ferromagnetic powder and a non-magnetic powder dispersed in a binder, respectively. In this magnetic recording medium, the coating composition containing the dispersed non-magnetic powder is prepared by dispersing the non-magnetic powder in a first binder component having a high adsorption of 2.2 mg/m2, and further dispersing the dispersion with a second binder component satisfying specific conditions.
A single use of such a highly adsorptive thermosetting binder resin as described in the above-mentioned publication, however, increases the viscosity of the coating composition, and the resulting non-magnetic layer still has an insufficient surface smoothness, although it is satisfactory in other properties.